1. Field of the Invention
The present invention relates to a rubber composition for a tire base tread for use in a base rubber layer in a tread rubber portion of a pneumatic tire.
2. Description of the Related Art
In recent years, a pneumatic tire has been increasingly demanded to be improved in low fuel consumption property. In order to meet such a demand, a terminal-modified diene rubber has been developed (see, Japanese Application Kokai No. 61-103904). The terminal-modified diene rubber is good in compatibility with a filler as a reinforcing agent such as carbon black or silica as compared with general unmodified diene rubbers. For this reason, it is capable of suppressing heat generation and improving the low fuel consumption property.
However, in order to improve the effect of the low fuel consumption property, it is necessary that the number of terminals is increased and that a larger amount of a modifier is added thereto. To this end, in general, a diene rubber is reduced in molecular weight to be used. Further, for the terminal-modified diene rubber, the dispersibility of the filler is improved by the effect of the modifier for the terminal. These cause reduction in hardness in a rubber composition for a tire base tread including a terminal-modified diene rubber added therein. This unfavorably leads to inferior driving stability.
When, to such a reduction in hardness, the enhancement in hardness is tried to be achieved with a conventional technique such as an increase in amount of the filler or a decrease in amount of the oil, the low fuel consumption property tends to be degraded. This results in a reduction of the low fuel consumption property improving effect by the use of the terminal-modified diene rubber.
On the other hand, there is also conceivable another measure in which the crosslinking density is increased by an increase in amount of sulfur as a vulcanizing agent for the enhancement in hardness. In that case, although the hardness and the low fuel consumption property are compatible with each other, the flexural fatigue characteristic is degraded due to the increase in crosslinking density.
Incidentally, for a rubber composition conventionally for use in the tread of a tire or the like, there is known the one obtained by adding 1,6-bis(N,N′-dibenzyl thiocarbamoyl dithio)hexane as a vulcanizing agent to a diene rubber, (see, Japanese Application Kokai Nos. 2001-2833, 2004-256792, 2005-263892, and 2006-45471). However, in the related arts, the vulcanizing agents. are used for controlling the changes with time with respect to heat, and improving the durability. Thus, unknown are the hardness recovering effect due to use in combination with a terminal-modified diene rubber having a specific molecular weight, and the effect of achieving the compatibility between the driving stability and the low fuel consumption property resulting therefrom.
Japanese Application Kokai No. 2002-36832 discloses a rubber composition using a diene rubber modified with a terminal modifier such as 3-glycidoxypropyl triethoxysilane, and 1,6-bis(N,N′-dibenzyl thiocarbamoyl dithio)hexane as a vulcanizing agent in combination. However, in this document, the rubber composition is used for the rubber reinforcing layer disposed on the inside of the sidewall portion or a bead filler. The effect of achieving the compatibility between the low fuel consumption property and the driving stability due to use for a base tread is not disclosed at all.